Contact Time Of Droplets Research Articles

Behaviors of microdroplets impinging on supercooled superhydrophobic microgrooves

Microdroplet impact icing is a common occurrence in high altitudes or cloud environments. There is a lack of in-depth understanding regarding the icing behaviors of microdroplets interacting with anti-icing microstructures. While the impact of microstructure pitch on delaying icing is extensively studied, the microstructure height, as a similarly technology-controllable structural parameter, has received comparatively little attention in research. This study investigates the impact dynamics of microdroplets on supercooled superhydrophobic microgrooves, utilizing the coupled level-set and volume of fluid and enthalpy-porosity technologies for precise simulation. The behaviors of microdroplets impact on supercooled surface manifest distinctive characteristics compared with ambient surface, particularly evident when We < 170 (βmax (maximum spreading diameter)/βMod (predicted spreading diameter) ∼ 0.8). Unlike flat surfaces, microgrooves have been shown to reduce βmax by 52%. Additionally, an increase in H/Do (ratio of microgroove height H and droplet diameter Do) leads to a diminishing trend in βmax, showing a reduction of 13%. The outcomes of microdroplet impact progress from complete bouncing, partial bouncing, bouncing with satellite droplet breakup, to deposition, sequentially as the We and surface supercooling degree escalate. Heightening the microstructure alters the transition point between partial bouncing and bouncing with breakup, thereby expanding the range of droplet bouncing (with thresholds being doubled). This effect is attributed to satellite droplet breakup, supported by simulation results suggesting that droplet breakup reduces interface curvature, thus facilitating bouncing. Moreover, microgroove structures have the potential to decrease the dimensionless contact time of droplets by 33% when comparing H/Do = 0.4 with H/Do = 0.1.

Liquid metal droplets impacting superhydrophobic surface in a viscous (non-oxidizing) environment

The hypothesis of the present research is the existence of distinct spatial-temporal characteristics of non-oxidized liquid metal (LM) droplets impacting a solid surface. To provide a quantitative claim to this hypothesis, we created a test matrix based on the well-known impingement regime map bounded by two dimensionless quantities—Weber number (We) and Ohnesorge number (Oh). The range of these quantities is from 10−2 to 102 (We) and 10−3 to 101 (Oh), leading to Reynolds number (Re) (≡We1/2/Oh) to vary from 10−2 to 104. The class of LMs opted for are post-transition metals—eutectic gallium alloys—due to their several desired practical features, such as low melting point, non-toxicity, and low vapor pressure. The research is conducted using numerical experiments performed using C++ OpenFOAM libraries. To ensure the reliability of the code, we tested our work with numerous impingement behaviors of fluids available in the literature. A plethora of droplet behaviors are reported, such as deposition, rebound, bubble entrapment, and splash. Several features of droplet impingement were critically examined, such as temporal spreading factor, maximum spreading factor, and contact time of droplets on the surfaces. Moreover, the conventional scaling laws regarding the impingement behavior of droplets were tested, with new ones proposed where deemed necessary. Furthermore, a distinct route for the entrapment of droplet is observed, caused by the bulging of LM droplet during the recoiling stage. Emphasis is made to form delineations for these impingement characteristics using dimensionless groups (i.e., We, Oh, and Re).

Numerical study of droplet impact on a superheated surface under an electric field based on perfect and leaky dielectric theories

AbstractThis paper investigates the hydrothermal behavior of leaky dielectric and perfect dielectric droplets impacting a superheated wall within a specific range of Weber numbers under an electric field. Through this investigation, we aim to provide a more comprehensive understanding of the dynamics involved in droplet‐superheated surface interactions under electric fields, which can be useful in various applications, such as the design of cooling systems and combustion chambers. The study utilizes the level‐set and ghost fluid techniques to capture the interface accurately. Under an electric field, different behaviors are observed during the impact process, depending on the electrical properties of the droplet. A perfect dielectric droplet experiences a reduction in spreading extent and an increase in contact time. However, no remarkable enhancement in total heat removal occurs in this case. For the leaky dielectric droplet exhibiting prolate deformation at the stationary state, increasing the electric field magnitude results in a slight decrease in the droplet spreading extent, while the droplet contact time and total heat removal from the surface increase. At an electric capillary number of 1.55E − 2 and a Weber number of 25, the enhancement in the contact time and total heat removal is about 43% and 15%, respectively. For the leaky dielectric droplet with oblate deformation at the stationary state, the spreading extent and total heat removal increase, with negligible changes in contact time. At the above‐mentioned electric capillary and Weber numbers, the enhancement in the spreading extent and total heat removal is about 7.5% and 15%, respectively.

Experimental study on contact time of a water droplet impact under controlled surface temperature

Bouncing droplets on superhydrophobic surfaces is one of the potential methods used for anti-icing. The surface supercooling is a significant parameter influencing the bouncing dynamic. A droplet impacting cold superhydrophobic surfaces is investigated via experimental methods. The influence of the surface supercooling and the Weber number on the impact dynamic is elucidated. Intriguingly, the surface supercooling shows no influence on the spreading time, and the initial retraction time as the heat exchange can be ignored in these processes, while it shows a strong influence on the late retraction time as it can lead to the wetting transformation. To further quantitatively describe the influence of surface supercooling, the relationships of the retraction rate in the late retraction are developed, considering the changes in the receding contact angle caused by the supercooling degree. Finally, the relationship of the contact time is established over a range of Weber numbers (We = 49.37–70.53), surface supercooling (ΔT = 15–24 °C), and droplet sizes (D0 = 2.2–3.2 mm). This work is the first to establish the relationship of the droplet contact time on cold superhydrophobic surfaces, which can provide a quantitative method to calculate the contact time for anti-icing.

Relaxation phase during droplet impact on superhydrophobic surfaces with high contact angle hysteresis

The droplet impact process on a solid surface is divided into a spreading phase where the droplet reaches the maximum deformation followed by a retracting phase. However, in the case of surfaces with high contact angle hysteresis, these two phases are connected by a relaxation phase where the contact angle changes from the advancing to the receding contact angle almost without motion of the contact line. Although the relaxation time can represent a significant part of the total droplet contact time, this relaxation regime has been less explored, especially for superhydrophobic surfaces due to the challenge of designing such surfaces with controlled wetting properties. Here, we show that for superhydrophobic surfaces with large contact angle hysteresis, the relaxation time can be comparable to the spreading and retracting time. Our results indicate that both the contact angle hysteresis and the capillary forces play a major role in defining the relaxation time and that relaxation time scales with the inertial–capillary time when using the droplet relative deformation as the characteristic length scale for this relaxation regime.

Synthesis and demulsification mechanism of an ionic liquid with four hydrophobic branches and four ionic centers

Stable emulsions can have numerous negative impacts on both the oil industry and the environment. This study focuses on the synthesis of two ionic liquids (via. PPBD and PPBH) with four hydrophobic branches and four ionic centers that can effectively treat oil-water emulsions at a low temperature of 40 °C. Their chemical structure was explored using Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance hydrogen spectra (1H NMR). The effect of temperature, PPBD and PPBH concentration, oil-water ratio, salinity and pH value on the demulsification efficiency (DE) of W/O emulsion was studied detailly and several commercial demulsifiers were also used for comparison. Results revealed that by adding 250 mg/L of PPBH in an E30 emulsion and leaving it for 120 min at 40 °C, the DE could reach 96.34%. Meanwhile, in an E30 emulsion (oil-water mass ratio of 3:7) with 250 mg/L of PPBD, the DE of 95.23% could be obtained at 40 °C for 360 min. Especially, the DE of PPBH could reach 100% in an E70 emulsion (oil-water mass ratio of 7:3) at the same conditions. Additionally, the demulsifier (PPBH) exhibited excellent salt resistance and outperformed some commonly used commercial demulsifiers. Several methods were utilized to investigate the potential demulsification mechanism, including measuring interfacial tension (IFT), three-phase contact angle (CA), droplet contact time, zeta potential, and observing samples under optical microscopy.

Directional rebound of compound droplets on asymmetric self-grown tilted mushroom-like micropillars for anti-bacterial and anti-icing applications

Directional liquid bouncing is of great significance for a variety of biological and industrial applications. Although surface engineering through 3D printing or self-assembly enables water droplets to rebound directionally, directional bouncing of compound droplets still poses a challenge due to their low surface tension. Moreover, preparation of superamphiphobic surfaces in an efficient and simple manner to accomplish this mission remains to be studied. Here, a tilted stepped mushroom-like micropillar surface (TSMMs), whose structural morphology can be created and tailored by a facile and direct femtosecond laser-induced asymmetric self-growing strategy, is designed to enable the oblique quasi-pancake bouncing of a low surface tension droplet (γ = 32 mN/m). Oblique quasi-pancake bouncing can not only reduce the droplet contact time (7 ms) but also effectively control the direction of droplet bouncing (the horizontal distance of 2.4 mm). Accompanying mechanistic analysis provides an in-depth understanding of the structure-droplet motion relationship. Finally, we demonstrate the applications of TSMMs in antibacterial and anti-icing scenarios. This work advances the investigation of directed droplet transportation, and paves new avenues in the fields of advanced biomedical and other industrial materials.

Molecular Dynamics Simulation of Droplet Impact on a Hydrophobic 3D Elastic Surface.

The phenomenon of droplets impacting elastic surfaces is common in nature and in many engineering applications. It has been shown that droplet impact on an elastic surface drastically reduces droplet contact time and hinders droplet spreading. However, most of the current studies are based on experiments, and the analysis of the influence mechanism of the elastic substrate on the dynamic behavior of droplets is not complete. In addition, the simulations of droplet impact on elastic substrates are mainly focused on 2D elastic films or vibrating rigid substrates, ignoring the effect of 3D elastic substrate deformation on the droplet dynamic behavior. Therefore, in this paper, we propose to model the droplet impact on a 3D hydrophobic elastic substrate using the molecular dynamics method. We find that droplet pancake rebound can substantially reduce the droplet contact time. Moreover, we record the conditions required for the pancake rebound of the droplet. Furthermore, we investigated the effects of the elastic modulus of the substrate and the initial velocity of the droplet on the droplet contact time, contact area, and spreading factor. This study further elucidates the influence mechanism of the elastic substrate on the dynamic behavior of the droplet and provides theoretical guidance for regulating the dynamic behavior of the droplet in related fields.

Whether contact time can evaluate the anti-icing properties of superhydrophobic surface - A research based on the MDPDE method

Impact freezing of droplets on a cold surface has attracted the attention of scholars. This process is vital for practical problems such as the anti-icing of aircraft wings and cables. Researchers have found that superhydrophobic surfaces can reduce the contact time of droplets and delay nucleation. However, whether contact time can evaluate the anti-icing properties of superhydrophobic surface needs further study. Considering the change in droplet temperature, we vary the dissipative force term of the conservative force in the many-body dissipative particle dynamics with the energy conservation (MDPDE) method. Then combined with the experimental results, a nucleation model of the recalescence rate related to the degree of undercooling is achieved. Finally, we obtain a droplet impact freezing model based on the MDPDE method for the first time, and the impact processes of supercooled droplets on the superhydrophobic surface with three different topological microstructures are explored. Effect of Weber number We on the critical adhesion temperature T* of micropillar surface is reversed when the solid fraction φ is different, and φ is the main factor affecting T*of the microcavity surface. When other conditions are the same, T* of the microridge surface is the lowest. And surprisingly, we observed that the contact time of droplets did not fully measure the anti-icing ability of the superhydrophobic surface.

Multiscale Superhydrophobic Zeolitic Imidazolate Framework Coating for Static and Dynamic Anti‐Icing Purposes

AbstractIce formation is a major challenge for engineering systems. Superhydrophobic surfaces constitute an effective approach to address this challenge. However, in addition to complex preparation methods, surface texture‐ and chemistry‐related shortcomings reduce their effectiveness. In this study, a functionalized metal–organic framework (ZIF‐8) based micro‐nano‐subnano scale coating (SuperHydrophobic Multiscale Coating – SHMC) with CA (contact angle) > 172°, rolling angle < 5°, and CAH (contact angle hysteresis) < 3° is developed and applied to metallic surfaces by spray coating. A fractal theory‐based model of contact angle is adapted to reveal its non‐wetting mechanism. SHMC extends the icing time by at least 300% and maintains its superhydrophobicity for > 30 icing/deicing cycles. The generated capillary pressure ranges within the multiscale coating are studied. The three‐phase contact line characteristics including contact times, contact diameters, and interfacial heat transfer during droplet impact are assessed. A numerical model is developed using dynamic contact angle physics for transient heat transfer during the impact. Compared to the plain surface, which leads to instant icing at 60 ms after impact, no icing is observed on the developed coating. At least an order of magnitude reduction in heat transfer rate during the droplet contact time is obtained with SHMC.

Regulation of droplet impacting on superhydrophobic surfaces: Coupled effects of macrostructures, wettability patterns, and surface motion

Superhydrophobic surfaces have shown great application prospects due to their excellent water repellency in many applications involving fluid–surface interactions. As a ubiquitous fluid–surface interaction phenomenon, droplet impacting dynamics has a crucial effect on the application of superhydrophobic surfaces. In this Perspective, we summarize the basic process of droplet impacting on superhydrophobic surfaces and introduce the two most concerned parameters that describe the droplet impacting dynamics, i.e., the maximum spreading coefficient and the contact time. We then review two improvement strategies for superhydrophobic surfaces: one is to construct macrostructures and the other is to set wettability patterns on the surface. The former strategy shows great potential in reducing the droplet contact time, and the latter one can accurately regulate the behavior of impacting droplets. The motion of superhydrophobic surfaces also changes the droplet impacting dynamics due to the additional aerodynamic effect or energy input, which arouses attention recently. However, only the individual influence of each factor (e.g., macrostructures, wettability patterns, or surface motion) on the droplet impacting dynamics has been focused in literature, so we write this Perspective to emphasize the importance and urgency of studying the coupled effects of these three factors.

Effect of the grooved structure on dynamic behavior of droplets impinging on the cylindrical superhydrophobic surface

The dynamic behavior of droplets impacting a grooved cylindrical superhydrophobic surface (a circular cylindrical shape with groove) was studied via experiments performed over D* (defined as the ratio of cylinder diameter D to initial droplet diameter D0) ranging from 4 to 9 and We ranging from 3 to 210. Parameters including the spreading, receding, splashing, and contact time were investigated. Five possible behaviors of the droplets, including complete rebound, partial rebound, no rebound, stretched breakup rebound and stretched breakup, were observed on the grooved cylindrical superhydrophobic surface. Based on the Weber number, the sites of these phenomena into three regions, namely complete rebound region (complete rebound), transition region (partial rebound, non-rebound), and splash region (stretched breakup rebound and stretched breakup) were divided. The results revealed that the diameter ratio D*, We, and grooved structure have a significant effect on the droplet morphology. The grooved structure hindered the droplet spreading and splashing in the perpendicular direction of the droplet and promoted the droplet spreading and splashing in the parallel direction. In addition, an equation for predicting the maximum axial and azimuthal spread diameter was proposed. Compared with the cylindrical surface, the grooved cylindrical surface could reduce the contact time of droplets within a certain range of We.

Understanding the impact dynamics of droplets on superhydrophobic surface

The dynamics of droplets impacting superhydrophobic surfaces is investigated using experiments and numerical simulations. The superhydrophobic surface is fabricated by growing the carbon nanotube forest on stainless steel mesh in order to perform experiments. To numerically model the surface, Kistler dynamic contact angle model is used. Depending on different physical properties such as drop Weber number, the inclination of the surface and Ohnesorge number, the impact outcomes are categorized into symmetric bouncing, asymmetric bouncing, breakup, and tail impingement. It is observed that with the increase in the inclination angle of the surface, the contact time of droplets on the surface decreases drastically. The energy budget of impacting drops is analyzed for the entire evolution cycle to delineate the process of drop deformation and its bouncing. It is observed that the droplets bouncing off the inclined surface carried more kinetic energy compared to those of their counterparts on horizontal surfaces. To address the bouncing phenomena, co-efficient of restitution of the drop is also explored. The inclination of the substrate is found to increase the co-efficient of restitution of the drop, which is directly related to the energy budget of the bouncing drop. Furthermore, the force exerted by a drop on such surfaces is also analyzed, showing two peaks, making superhydrophobic surfaces more prone to damage in comparison to hydrophilic surfaces, which showed only a single peak. It is observed that the magnitudes of the peak forces are smaller for oblique impact as compared to horizontal impact.

Coalescence dynamics of nanofluid droplets in T-junction microchannel

The effect of nanoparticles on droplet coalescence in microchannel was investigated experimentally. The interaction between acid-functionalized nanoparticles and polymers with complementary terminal functional groups facilitates the adsorption of particles at the oil–water interface. Three flow patterns were observed: squeezing coalescence, decompression coalescence and non-coalescence. The presence of nanoparticles significantly reduces the droplet coalescence percentage, and stable droplets could be obtained when the particles concentration increased to 1%. Furthermore, it was found that the film drainage time of squeezing coalescence was independent of the nanoparticles concentration. However, the film drainage time of decompression coalescence increases with nanoparticles concentration. Taking the influence of nanoparticles on the film drainage time into consideration, a new prediction model was proposed. Coupling the droplet contact time with the film drainage time, the critical capillary number for droplet coalescence was obtained. This work could provide important data and technical support for the application of nanoparticle-stabilized emulsions.

Lattice Boltzmann simulation of droplet impact dynamics on superhydrophobic surface decorated with triangular ridges

The rapid rebound of droplets impacting on superhydrophobic surfaces is used in a wide range of applications such as anti-icing, anti-forcing and self-cleaning. In this study, the droplet impacting a superhydrophobic surface equipped with macroscopic triangular ridges is simulated using the three-dimensional lattice Boltzmann method (LBM) and focus on the effects of Weber number (We) and the number of ridges on the dynamic behavior and contact time. For the single ridge, as We increase, the droplet gradually form a ring shape and the splits. The appearance of ring shape leads to a rapid decrease in contact time t * , which is because an opposite fluid movement in the inner and outer edges collides with each other and generate an upward velocity that facilitate the droplet bouncing. Base on the single ridge, the structure of cross ridges and quadruple ridges are proposed to research the effect of different number of ridges to the contact time. It can be found that the contact time and phenomenon between cross and quadruple ridges are similar, and the contact time decreases by about 50–66.7 % substantially and has a limited time compared to the single ridge, although there is no break or ring forming before they rebound. Therefore, the crossed ridges(collective name for cross and quadruple ridges) structure helps to reduce the contact time of droplets impacting the superhydrophobic surface, but an excessive number of ridges should be avoided.

Droplet impact on a heated porous plate above the Leidenfrost temperature: A lattice Boltzmann study

In the past few decades, the droplet impact on a heated plate above the Leidenfrost temperature has attracted immense research interest. The strong hydrophobicity caused by the Leidenfrost effect leads to the droplet bouncing from a flat plate at a given contact time predicted by the classical Rayleigh theory. Numerous investigations were conducted to break the theoretical Rayleigh's limit to reduce the interfacial contact time. Recently, a droplet was observed to form a pancake shape and bounce as it impacted nanotube or micropost surfaces above the Leidenfrost temperature. This led to a significant reduction in droplet contact time. However, this unique bouncing phenomenon is still not fully understood, such as the influence of the plate configuration and the relationship between the droplet rebound time and evaporation mass loss. In this study, we carry out a numerical study of the droplet impact dynamics on a heated porous plate above the Leidenfrost temperature, using a multiphase thermal lattice Boltzmann model. Our model is constructed within the unified lattice Boltzmann method framework and is first validated based on theoretical and experimental results. Then, a comprehensive parametric study is performed to investigate the effects of the impact Weber number, the plate temperature, and the plate configurations on the droplet bouncing dynamics. Results show that higher plate temperature, larger Weber number, and smaller pore intervals can accelerate the droplet rebound and promote the droplet pancake bouncing. We demonstrate that the occurrence of the pancake bouncing is attributed to the additional lift force provided by the vapor pressure due to the evaporation of liquid inside the pores. Moreover, the droplet maximum spreading time and maximum spreading factor can be described by a power law function of the impact Weber number. The droplet evaporation mass loss increases linearly with the impingement Weber number and the plate opening fractions. This study provides new insights into the Leidenfrost droplet impingement on porous plates, which may potentially facilitate the design of novel engineering surfaces and devices.

Design principle of ridge-textured superhydrophobic surfaces for inducing pancake bouncing

Reducing contact time of droplets impacting on solid surfaces plays an important role in various applications such as anti-icing and energy harvesting. Pancake bouncing on superhydrophobic surfaces decorated with arrays of pillars or ridges can remarkably reduce the contact time. However, the trigger of pancake bouncing is rigorous, and precise controls of impact conditions and surface structures are the significant precondition for utilizing pancake bouncing. Here, we numerically investigate the dynamic process of droplets impacting on ridge-textured superhydrophobic surfaces, and theoretically establish the design principle for inducing pancake bouncing. Compared with conventional bouncing on flat superhydrophobic surfaces, the contact time of pancake bouncing on ridge-textured superhydrophobic surfaces is effectively reduced by 60–70%. The transition of bouncing depends on the time for the penetrated liquid to drain out from surface structures and the energy of droplets when drainage finishes. Correspondingly, time and energy demands are needed to be satisfied to induce pancake bouncing, and the two demands compose the design principle. Initial parameters are integrated into two parameters only related to droplets and surfaces, respectively. The design principle accurately determines the proper structure parameters for pancake bouncing under different impact conditions, which provides useful guidance for the design of superhydrophobic surfaces

Dynamics of droplet impact on a superhydrophobic disk

An experimental study is performed to investigate the effect of tangential velocity on the dynamics of a water droplet impacting on a spinning superhydrophobic surface. It is revealed that an increase in the tangential velocity results in the spreading of a droplet from symmetrical to asymmetrical shape on the superhydrophobic surface. Moreover, depending on the impact and tangential velocities, three behaviors are observed: bouncing, symmetrical splashing, and asymmetrical splashing. In the bouncing regime, it is found that the droplet contact time is independent of impact velocity and decreases as the tangential velocity increases. However, the maximum spreading diameter in this regime is a function of both the impact and the tangential velocities. Furthermore, a splashing threshold defined as WeRe1/21−kRe−1/2V/U2=K is introduced to estimate the transition between the bouncing, symmetrical splashing, and asymmetrical splashing regimes. It is revealed that the value of K in the present work (i.e., superhydrophobic spinning disk) is approximately 60% less than the K value obtained by other researchers for the case of aluminum spinning disk. Moreover, two values are found for k to define the boundaries between these three observed regimes.

Air Cushion Storing Energy Promoting Droplets Retraction and Flow on Engineering Porous Bionic Lotus Surfaces

AbstractPorous bionic self‐cleaning surfaces with low contact time of droplets show good potentials in anti‐icing, drag reduction, antifouling, etc. However, the reason of asymmetric and fast retraction on inclined surfaces after droplet impact is not clear. Here, it is reported that the fast retraction is mainly ascribed to the “air cushion” in porous surface acting as “energy reservoir” that stores excess kinetic energy of droplet during spread and returns it back promoting droplets retraction with contact time 20–40% off. Besides, the pinning effect and wetting state transition result in the asymmetric morphology evolution and suddenly stretch along tangential. A physical model of droplet asymmetric retraction including the influence of dynamic wetting angle fDCA, pinning effect fPin, and air cushion fAir is innovatively proposed to describe droplet morphologic evolution. The fundamental understanding of droplets impact dynamic on inclined surfaces is beneficial for engineering applications of extremely wettable surfaces.

Reducing Contact Time of Droplets Impacting Superheated Hydrophobic Surfaces.

Reducing the contact time (tc ) of a droplet impacting a solid surface is crucial in various fields. Superhydrophobic (SHB) surfaces are used to reduce tc at room temperature. However, at high temperatures, SHB surfaces cannot achieve tc reduction because of the failure of the coating materials or the Leidenfrost (LF) effect. Therefore, a surface that can suppress the LF effect and reduce tc at high temperatures is required. To create such a surface, a double-reentrant groove (DRG) array surface with an overhanging structure on top of the microgrooves is developed. The overhanging structure renders the surface hydrophobic (HB). Despite its HB nature, the DRG surface's LF point (LFP) is observed at ≈530°C, which is higher than the LFP on other HB surfaces.Moreover, a tc smaller than the inertia-capillary limit on the DRG surface is observed at between 400 and 500°C. Accordingly, the DRG surface is currently the only HB surface for tc reduction at high temperatures. The DRG surface avoids the limitation of low LFPs observed on HB surfaces. Due to its HB properties, the DRG surface is determined to exhibit self-cleaning characteristics and can be used in various applications at high temperatures.